Cessna 172 Maintenance Manual page 21

of the galvanic cell. The degree of attack depends on the relative activity of the two surfaces; the
greater the difference in activity, the more severe the corrosion. Relative activity in descending
order is as follows:
(a) Magnesium and its alloys.
(b) Aluminum alloys 1100, 3003, 5052, 6061, 220, 355, 356, cadmium, and zinc.
(c) Aluminum alloys 2014, 2017, 2024, and 7075.
(d) Iron, lead, and their alloys (except stainless steel).
(e) Stainless steels, titanium, chromium, nickel, copper, and their alloys.
Graphite (including dry film lubricants containing graphite).
(f)
D. Intergranular Corrosion.
(1) Selective attack along the grain boundaries in metal alloys is referred to as intergranular
corrosion. It results from lack of uniformity in the alloy structure. It is particularly characteristic
of precipitation hardened alloys of aluminum and some stainless steels. Aluminum extrusions
and forgings in general may contain nonuniform areas, which in turn may result in galvanic
attack along the grain boundaries. When attack is well advanced, the metal may blister or
delaminate which is referred to as exfoliation.
E. Stress Corrosion.
(1) This results from the combined effect of static tensile stresses applied to a surface over
a period of time.
stresses approaching the yield point, and with increasing temperature, exposure time, and
concentration of corrosive ingredients in the surrounding environment. Examples of parts which
are susceptible to stress corrosion cracking are aluminum alloy bell cranks, landing gear shock
struts with pipe thread-type grease fittings, clevis points, and shrink fits.
F. Corrosion Fatigue.
(1) This is a type of stress corrosion resulting from the cyclic stresses on a metal in corrosive
surroundings. Corrosion may start at the bottom of a shallow pit in the stressed area. Once
attack begins, the continuous flexing prevents repair of protective surface coating or oxide films
and additional corrosion takes place in the area of stress.
3. Typical Corrosion Areas
A. This section lists typical areas of the airplane which are susceptible to corrosion. These areas should
be carefully inspected at periodic intervals to detect corrosion as early as possible.
(1) Engine Exhaust Trail Areas.
(a) Gaps, seams, and fairings on the lower fuselage, aft of the engine exhaust pipe(s) are
typical areas where deposits may be trapped and not reached by normal cleaning methods.
(b) Around rivet heads, skin laps and inspection covers on the airplane lower fuselage aft of
the engine exhaust pipe(s) should be carefully cleaned and inspected.
(2) Battery Box and Battery Vent Opening.
(a) The battery, battery cover, battery box, and adjacent areas, especially areas below the
battery box where battery electrolyte may have seeped, are particularly subject to corrosive
action. If spilled battery electrolyte is neutralized and cleaned up at the same time of
spillage, corrosion can be held to a minimum by using a baking soda solution to neutralize
the lead acid-type battery electrolyte. If baking soda is not available, flood the area with
water.
Stainless Steel control cables.
(3)
(a) Checking for corrosion on control cables is normally accomplished during the preventative
maintenance check. During preventative maintenance, broken wire and wear of the control
cable is also checked.
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
In general, cracking susceptibility increases with stress, particularly at
© Cessna Aircraft Company
51-11-00
Page 3
Jun 1/2005